The present invention relates to a optical pickup for optical recording and information read in a CD (compact disc) player and the like, and in particular, to an integrated optical pickup using an optical waveguide.
FIG. 1 is a figure showing the principle (central aperture method) for read of an optical disc using a conventional optical pickup.
As shown in the figure, a light that is emitted from a semiconductor laser 11 passes through a half mirror 12 and links a spot on an optical disc 14 by a (objective) lens 13. When the spot strikes a pit, there is diffraction by the pit and the oth-order light that is reflected by the optical disc 14 returns as it is to the aperture of the lens 13 but the .+-.1st order diffracted lights are phase shifted in proportional to the depth d of the pit, and return to offset regions 16, 17 from the aperture of the lens 13. Because of this, at the lens aperture, there is interference by the portion of the oth-order light and the .+-.1st order diffracted light that overlap, and there is a drop in the intensity because of the phase difference between them. This description is for the case where the spot is on the pit but in cases when there is no pit and the spot is on a mirror surface of the optical disc 14, there is practically no diffracted light and the intensity of the light reflected to the lens 13 is unchanged. In both cases however, the reflected light that enters the lens aperture is reflected by the half mirror 12 and focuses on a photodiode 15. However, since as has been described above, there is a change in the intensity of the reflected light in the lens aperture because of the presence or absence of a pit and so an electrical signal corresponding to that intensity change is obtained as the photodiode output.
In addition, FIG. 2 is a diagram showing an example of a conventional integrated optical pickup. To FIG. 1, the objective lens 13 is replaced with a focusing grating coupler 13A, and the half mirror 12 is replaced with a grating beam splitter 12A. A light guiding layer 20 is formed on a semiconductor substrate 19. The principle of reading is the same as that shown in FIG. 1.
There are the following problems in the conventional optical pickup. Signal detection by a conventional optical pickup does not use the portion of the .+-.1 diffracted lights which do not enter the lens aperture, and the availability of the reflected light decreases.
With signal detection by the conventional optical pickup, a pit on the disc is detected as the signal which is the sum of the outputs of a plural number of photodiodes and so it is not possible to remove the common-mode optical and electrical noise in the same phase and this results in a poor signal-to-noise ratio.
In order to eliminate this problem, there has been proposed in U.S. Pat. No. 3,913,076 a method that detects the oth order light and the diffracted light separately and takes the difference of the two to detect pits. With this method, there are sensors for the respective detection of the oth order light and the diffracted light disposed on the inside of the far field of the read light and so there are many restrictions due to the shape of the sensors and their position of placement, and in reality, there is the problem that there is little design freedom with respect to the configuration of the optical pickup.